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Science of reproduction > Antral follicugenisis > Flashcards

Antral follicugenisis Flashcards

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1
Q

What is the theca and what are its functions?

A
  • An envelope of connective tissue
  • differentiates into theca interna & externa containing vascular tissue, immune cells and matrix factors
  • Theca is critical for maintaining structural integrity of follicle and delivering nutrient to avascular GC layer
  • Has LH receptors
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2
Q

What is important for preantral to antral progression?

A

Formation and differentiation of theca

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3
Q

Why is the theca important for preantral to antral progression?

A
  • Neo-angiogenesis from theca, hence follicle interaction with systemic endocrine factors
  • Acquisition of steroidogenic function as theca is steroidogenic – theca cells make androgens
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4
Q

How many sources do the theca cells derive from and what are they?

A

from 2 different sources in the embryonic gonad

  • Mesenchymal (from mesonephros) cells become steroidogenic cell
  • Stromal cells (indigenous to medullary region) become fibroblasts, perivascular smooth muscle cells and interstitial ovarian tissue
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5
Q

What is the antrum?

A

a single, large, fluid-filled cavity:

o Contains follicular fluid formed as exudate of plasma containing secretory products of oocyte & GC

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6
Q

How does antrum formation happen?

A
  • When the follicle reaches a diameter of 200-400µm, surrounded by a vascularized theca, it becomes subjected to circulating influences.
  • Fluid-filled spaces appear between the granulosa cells which soon coalesce together to form a single, large, fluid-filled cavity.
  • As the fluid volume increases the follicle continues to expand greatly in size.
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7
Q

What is important for antrum formation?

A

o KL and Cx37 essential for antrum formation in lab animals – as k/o of these genes result in no antral follicles at all

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8
Q

What is the follicular fluid formed of?

A
  • formed by filtration of thecal blood, its composition is different from plasma as contains secretory products of oocyte and granulosa cells
  • As the fluid volume increases the follicle continues to expand greatly in size.
  • Granulosa cells and Follicular fluids both increase as the follicle grows and develops
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9
Q

What is the theca externa?

A
  • Concentrically arranged smooth muscle cells
  • innervated by autonomic nerves
  • have lymphatic vessels
  • important during ovulation
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10
Q

What is the theca interna?

A
  • Steroid producing cells
  • Contain LH-R and insulin-R
  • is richly vascularised
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11
Q

What is COC (cumulus-oocyte complex)?

A

an oocyte surrounded by specialized granulosa cells, called cumulus cells

  • they interact with oocytes via gap junctions
  • Have no LH-R
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12
Q

What happens to COC during the LH surge?

A

COC become expanded and have a sticky mucified nature as a response to the LH surge

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13
Q

How is COC expanded?

A

GC produce EGF-like ligands that bind LH and allow for secretion of hyaluronan and a complex of hyaluronan cross-linking proteins that cause expansion of COC

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14
Q

Why is the intercycle rise of FSH important?

A
  • crucial for the recruitment of antral follicles into the menstrual cycle’
  • needed for the selection of antral follicles
  • needed for the selection of the dominant follicle
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15
Q

What is the two-cell, Two-Gonadotrophin Concept?

A
  • The HPG axis acts to control antral follicle growth at this stage
  • In response to LH, theca expresses key steroidogenic enzymes to make androgens from cholesterol – those enzymes are crucial
  • Likewise granulosa cells respond to FSH by up-regulating aromatase and 17β-HSD to make oestrogens
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16
Q

What stimulates and maintains the expression of FSH-r?

A

Activin

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17
Q

Effects of FSH in the dominant follicles

A
  • increase in granulosa cell proliferation
  • increase in aromatase activity
  • induce and maintain FSHr
  • induce and maintain LHr
  • interact with paracrine factors
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18
Q

What else interacts with FSH?

A

AMH and inhibin B

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19
Q

What kind of receptor is FSH-r?

A

G-protein coupled receptor

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20
Q

What happens when FSH binds to FSH-r?

A
  • It causes a downstream cascade signalling pathway
  • produces cAMP to activate PKA
  • leads to differentiation of granulosa cells and increase follicular fluid, gap junctions, LHr, FSHr and P450 aromatase
  • FSH gives positive feedback – stimulates FSHr genes, its aided by activin
21
Q

What is the effect of androgens acting on granulosa cells?

A

Androgens act on granulosa cells (paracrine interaction) to upregulate androgen receptor & FSHR

22
Q

What does AMH do in the dominant follicle?

A

• AMH (produced by GC of small antral follicles) acts as a brake on FSH recruitment of antral follicles by:

o decreasing FSH sensitivity
o decreasing FSH-stimulated aromatase expression

23
Q

Why is counterbalancing the effects of AMH, FSH and androgens important?

A

to ensure against
o premature depletion of PF pool and/or
o premature selection of follicles by FSH

24
Q

How is the dominant follicle recruited?

A

Each follicle has a different amount of FSH required to recruit one follicle and the follicle with the lowest threshold will be recruited within that environment

25
Q

What will a dominant follicle have which helps it get recruited?

A

o lower threshold of FSH required
o Increased numbers of FSH-R
o FSH-r coupled more effectively to down-stream signalling – exert FSH actions more effectively

26
Q

What are the actions of FSH?

A

• Growth and Oestradiol production
o increases cell division
o Increases in size
o increased aromatase

  • Dominant follicle selection
  • Increases area of theca vasculature – more open to circulating influences like insulin, growth factors and gonadotrophins
  • FSH acts on the selected follicle to switch on the LHr gene
27
Q

How does the dominant follicle survive the fall in FSH

A
  • has increased sensitivity to FSH&raquo_space;> increased FSH receptors
  • has increased numbers of granulosa cells so more oestradiol
  • acquisition of LH receptors - the LHR gene is switched on by FSH
  • possible involvement of insulin-like growth factors 1&2 (IGF-2 particularly important in humans)
28
Q

What does IGF-2 do in animals?

A

• FSH effects; stimulates androgen output and hence oestrogen

29
Q

What suppresses IGF activity?

A

IGFBP (IGF-binding protein)

30
Q

How is IGF made?

A
  • IGF cleaved from IGFBP by PAPP-A (pregnancy-associated plasma protein A)
  • PAPP-A expression high in DF
  • Thought that other AF in cohort may have higher levels of IGFBP hence preventing co-stimulatory effect of IGF & FSH
31
Q

What happens to the dominant follicle in the pre-ovulatory phase?

A
  • Grows a lot

- Increase in follicular fluid and granulosa cells

32
Q

What happens to the rest of the follicles that were not selected?

A

They die off

33
Q

What is the role of LH in Antral Follicles?

A
  • LH acts on theca and on the granulosa only after DF has been selected
  • Induces progesterone receptors in the granulosa of the DF
  • Increases the functions of the theca enzymes
  • increases growth and steroidogenesis in dominant follicles
  • Causes expansion of COC
  • causes withdrawal of gap junctions between gc and oocyte leading to the resumption of meiosis
34
Q

What does LH binding to LH-r in the theca interna do?

A
  • LHr is a GPCR, it makes cAMP
  • leads to the formation of steroids from cholesterol through the activation of enzymes that catalyse the steroid formation
35
Q

What increases the actions of LH?

A

Insulin

36
Q

What do LH and FSH have in common?

A

FSH have same 2nd messenger - cAMP

37
Q

How does the cell distinguish between cAMP produced by LH and FSH?

A
  • FSH produces low cAMP levels
  • LH produces high cAMP levels
  • linked to the difference in density of FSHr & LHr (LHr is more thought to be more prevalent than FSHr or LHr more effectively coupled to cAMP generation)
38
Q

What else will be needed to support follicle growth?

A
  • Angiogenic factors stimulated by primarily by androgens but also oestrogens – theca, gc, stroma all involved
  • Basic fibroblast growth factor (bFGF) - it’s a Mitogen for endothelial cells which form the blood vessels, most potent angiogenic factor
  • Vascular endothelial growth factor (VEGF) - it’s a mitogen for endothelial cells which form the blood vessels, enhances vascular permeability
  • Ovarian lymphatic vessels recruited to theca and stroma layers around growing follicle, under control of VEGF-R3
39
Q

Why do we need angiogenesis?

A

Constant re-modelling to allow for growth of follicle (2-20mm) through the ovarian tissue, angiogenesis of CL, tissue repair etc.

40
Q

How do androgens act on the DF?

A
  • Androgen binds to its (nuclear) receptors
  • they translocate to the nucleus and bind to ARE (androgen response elements) on the HIF-1 gene
  • this indues HIF-1 expression and that binds to HIF response elements (HRE) on the VEGF gene and that induces production of VEGF
  • VEGF binds to receptors on the endothelial cells and induces proliferation
41
Q

When is AMH secretion maximal?

A
  • during small antral follicle stages and decreases to undetectable levels later
  • Hence serum AMH reflect small AFs
42
Q

What do low numbers of antral follicles signify?

A

• Low numbers of antral follicles are a sign of ovarian ageing – antral follicle count and AMH serum levels checked for patients that can’t conceive

43
Q

How can AMH be used to predict fertility?

A

• Hence serum AMH reflect small AFs
• The number of antral follicles (AFC) in the early follicular phase correlates with numbers of growing follicles only
• This change in follicles and AMH is observable earlier than a rise in FSH serum level
•
However in women with PCOS, AMH levels are high but they’re still less fertile

44
Q

What is used to determine the functional ovarian reserve?

A

AFC & serum markers FSH, AMH, E2 and Inhibin B used to determine “functional ovarian reserve”, but does not indicate true PF reserve ie what you are born with

45
Q

What is Premature Ovarian Failure/Primary Ovarian Insufficiency?

A
  • POI is clinical term for premature ovarian failure
  • Defined as ovarian dysfunction <40yrs → oligomenorrhoea or amenorrhoea
  • Overarching feature is infertility resulting from accelerated depletion or reduced follicle reserve.
46
Q

What can cause POI?

A

o Environmental genotoxins induce DNA damage
o Mutations in genes like BRCA1/2
o Altered hormonal signalling
o Chromosomal defects e.g. Turner’s syndrome (XO) → have streak ovaries
o Autoimmune diseases including thyroiditis & Addison disease

47
Q

treatments for POI

A

cryopreserving ovaries
biopsies
egg donations

48
Q

How many types of 3D culture systems exist for follicles?

A 
2 types:
o Collagen
o Alginate (product of seaweed) – able to produce antral follicles from small secondary preantral follicles in human and mokeys
49
Q

requirement of a 3D culture systems for follicles?

A
  • Hydrogel matrix to support follicle so that contact maintained between oocyte and gc
  • Also to be permeable to media but also rigid to support the follicle
  • Modifiable in terms of rigidity as follicle needs space to grow

Science of reproduction (32 decks)

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